Technical Field
The present disclosure relates to a method and an imaging system for generating spectrally different X-ray images with an X-ray source and an X-ray detector. More particularly, the present disclosure relates to including a filter providing different spectral filtration within an X-ray system in order to produce a spectrally modulated beam such that neighboring pixels of the X-ray detector receive different spectra, and using this spectral information to perform means of spectral X-ray imaging.
Description of Related Art
Computed tomography (CT) is the science of recovering a three-dimensional representation of a patient or object by utilizing projection views with different orientations. From this volume, e.g., two-dimensional cross-sectional images can be displayed. CT systems typically include an X-ray source collimated to form a cone beam directed through an object to be imaged, i.e., a patient, and received by an X-ray detector array. The X-ray source, the cone beam, and the detector array may be rotated together on a gantry within the imaging plane, around the imaged object.
However, the X-ray radiation imposes unwanted effects. In the medical imaging domain, an unwanted effect may be the radiation dose that a patient receives, as it may induce damage to cells and genes. As a further unwanted effect, the interaction of X-ray radiation with matter imposes scattered X-ray radiation, which adds in the detector to the signal of interest, i.e., the signal of the primary radiation. As a most obvious method to reduce the unwanted effects, measures are taken to limit the amount of total X-ray exposure to a minimum, which is required to acquire images.
To reduce the negative effects, three elements are used to form the cone beam. First, a collimator defines a cone shape such that the cone beam covers exactly the whole detector area in order that each detector imaging element (denoted herein as a “pixel”) is exposed to the beam, but the overlap to the non-detector area is reduced to a minimum. Second, a bowtie-shaped device, known as a “beam shaper,” “bow tie” or sometimes also as a “wedge,” is placed in the path of the X-ray beam. The wedge, functioning as an X-ray attenuation filter, is generally made of a light metal, such as aluminum, or a synthetic polymer, such as Teflon, having an X-ray absorption spectral characteristic near that of water, and, hence, the human body.
The wedge is intended to compensate for the variation in thickness of the imaged body. The X-rays that pass through the center of the imaged body, normally the thickest part, are least attenuated by this filter, whereas the X-rays that pass through the periphery of the imaged body, normally the thinnest part, are more attenuated by this filter. The result of this selective attenuation is a better distribution of the X-ray dose.
This allows, on one hand, for a total dose reduction for the scanned patient. On the other hand, the X-rays impinging on the detectors have a less spatially varying intensity profile. The wedge may therefore allow use of more sensitive X-ray detectors, thus reducing the total dynamic range of x-ray intensities to be detected. Finally, as a third element to reduce negative effects, a spatially homogeneous filter (typically in the form of a metal plate, e.g., made of copper) is induced to absorb mainly the low energy components of the spectrum. The low energy components of the plain X-ray spectrum are typically that strongly attenuated by an object or a patient that they do not significantly contribute to a measured signal. Thus, the filter reduces the total dose a patient is exposed to with an acceptable reduction of the acquired detector signal.
Next, it is also desired to reduce scattered X-ray radiation to a minimum, as its intensity overlays to the primary intensity and therefore induces image artifacts due to a higher intensity measured. It is therefore desirable to develop methods to determine the amount of scattered radiation so as to correct the measured radiation for the scattered radiation signal. Typically the scattered radiation cannot easily be accessed as it is a priori not distinguishable from the primary radiation. Further, it is also difficult to determine it from the whole context of an acquired image as scattered radiation is related to the scanned patient geometry in a complex manner.
Special aspects of CT are spectral methods commonly termed as dual energy CT, multi-energy CT, or spectral CT. The common characteristic of all these methods is that they take use of the fact that different materials attenuate X-rays differently with respect to the energy of the X-ray photons. Consequently, the acquisition of CT projections with different weightings put on the X-ray photon energies provides additional (3D) information, not only of the material density, but also of the chemical composition. In other words, if it is possible to scan an object volume with data sets representing different spectral weightings, it becomes possible to apply mathematical methods to generate a 3D data volume representing different physical or chemical properties. Commonly known examples for such properties are the ratio of bone mineral density to soft tissue density, or the visualization of the presence of contrast agent content like that of iodine, barium, gadolinium, gold or other chemical elements. Other examples are the generation of separate 3D volumes of material densities containing water-like tissue, bone mineral, and/or K-edge contrast material. All these methods work better the stronger the spectral separation of the acquired projections is. Dependent on the chosen methods, the number of separated physical/chemical properties or the number of distinguishable materials also depend on the number of different spectra used for the projection generation.